Single definition of genetic diversity useable for all purposes is unavailable (UNEP, 1995). Nature’s knowledge lies in DNA of living cells (Meadows, 1990). Observed variations in living world are due to variations in sequences of four base pairs of DNA and their interactions with environment.
Pool of genetic diversity of a species exists at three levels viz., genetic diversity within individuals, often referred to as heterozygosis; genetic diversity between individuals within a population and genetic variation between populations.
Genetic diversity is heritable variations within and between populations. New genetic variations are produced in individuals of a species through gene or chromosomal mutations. In sexually reproducing organisms, recombination also contribute in genetic diversity.
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Genetic diversity in a population enables natural evolutionary changes to take place. Rate of such changes is proportional to amount of available genetic diversity. Conservation of species diversity is dependent on degree of genetic diversity within each species (Woodruff, 1989). Because genes can result in competitive advantage, genetic diversity at individual and population levels ultimately determines ability of species to survive and adapt to environmental conditions.
This measure of fitness is fundamental to process of natural selection and ultimately to evolution. In theory then, higher genetic diversity offers more opportunities for survival and vice versa (Ledig, 1986; Cockburn. 1991).
Under natural conditions, a degree of genetic diversity is retained as populations of a species interact over time and space. This is referred to as gene flow. When viable populations become isolated for long periods, adaptations to local conditions can eventually result in speciation (Primack, 1993).
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This process, known as adaptive radiation, contributes to an overall increase in species diversity. Yet, when only small populations are involved, isolation can reduce genetic diversity and eventually species diversity as well. In such cases, diminished gene flow and increased likelihood of inbreeding may lead to a reduction in overall fitness of population, and eventual risk of extinction.
These reveal the mechanisms behind theory of island biogeography and potential legacy of habitat fragmentation. Genetic diversity is essentially a source for species own survival and future evolution.
Genes controlling fundamental processes like photosynthesis and respiration are highly conserved across different taxa and generally exhibit little variations (Groom bridge, 1992). Genetic diversity of microorganisms is higher than macroorganisms because the former has originated first and existed on earth for a longer time. Of the 95 Phyla of living organisms (Margulis and Schwartz, 1988), 52 belong to microorganisms excluding viruses. A total of 159 mutant lines in Chlamydomonas reinhardii and more than 3000 mutants in Neurospora crassa (Woese et al., 1990) exemplify genetic diversity of microbes.
Most genetic variability within species over a single generation occurs due to shuffling of homologous chromosomes, crossing over and fusion of gametes during sexual reproduction.
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Mutations are rare, occurring once in 105 to 106 genes per individual in a generation. Evolution would stop without mutation, although mutation does not take place in anticipation of environmental demand. It simply happens, bringing structural and functional changes of organisms. Environmental conditions determine whether such changes are beneficial or harmful or neutral for present as well as the future.
When a single species population invades a number of new habitats and evolves due to differing environmental pressures, than many new species would evolve in a relatively short period through adaptive radiation.
This happens due to superior adaptation in a species that enables to displace a less adapted species from a variety of habitats. Natural selection favours spread of a mutation or not. It can’t directly detect an organism’s genotype rather acts on phenotypes.
Mutation, migration, selection and drift, individually and collectively, alter allele frequencies and bring about evolutionary divergence and cause the formation of species depending upon ecological diversity.
Wild relatives (weedy crop relatives) and species contribute substantially to expansion of genetic base of cultivated taxa. Hence, they are invariably used to breed and improve the latter (Harlan, 1976; Prescott-Allen and Prescott- Allen, 1988; Stalker, 1980). Wild relatives often provide genes that are not available in domesticated plants.
Such genes afford resistance to diseases/ pests and other environmental stresses. Such resistant genes have been acquired high wild relatives through their long period of co-evolution with microbes/pests as well as survival for a long time under stressed environments of various sorts. Many of these resistance genes control specific resistance.
Wild relatives of cultivated plants form part of primary, secondary and tertiary gene pools (Harlan and de Wet, 1971). These gene pools are also known as Genetic Resources Profiles of crop species (Smartt, 1990).
Primary gene pools (GP1) represent the true biological species including all its cultivated (cultigens), wild and weedy forms; hybrids among these forms are fertile and gene transfer to crop is simple, direct and poses no problem.
Most primary gene pools show at least 80% genetic closeness to crop species. Secondary gene pools represent the species group that can be artificially hybridized with crop but gene transfer may not be easy.
Hybrids, if produced, are usually weak or partially sterile. Secondary gene pools show around 60% closeness to crop species Tertiary gene pools (GP3) include all species that can be crossed to crop species but with some difficulty.
These gene pools show around 40% closeness to cultivated specie. There is also a quaternary gene pool; its constituents are incompatible with related species (Krishnamurthy, 2004).